This is probably a very trivial for some of you. But sometimes a name not representing the function become a question, just like this Collective Control, which its function is to pitch up the blade when it pulled up, and vice versa to pitch down when it pulled down. Why that device is named Collective Control? Why not another name, i.e., Pitch Control, Lever Control, etc., which is seem more representative. And why does it located on the left side of the pilot seat?
"Collective" is short for "Collective Pitch Control", which, you know, controls the pitch of the blades collectively.
It's on the left side because the really fine motor control required of flying a helicopter with the cyclic is best suited to the right hand, and 90% of the population is right handed.
A lot of machines (Hughes and others are exceptions) are flown from the right seat because the collective hand is the "free-est" you might say, since you can let go of collective but are not supposed to let go of cyclic. Flying from the right allows you to work all the switches in the cockpit without taking your right hand off cyclic. On left seat solo machines like Hughes 300/500s or Bell 47s or Enstroms, you have to switch hands all the time any time you want to work a cockpit control.
Firstly, it does not control the pitch of the helicopter, that can be achieved using the cyclic control.
Secondly, it is named collective because it changes the pitch of of all the main rotor blades collectively, as in, at the same time and by the same amount.
This is different from the cyclic control which adjusts the pitch of the blades as a function of where they are in their cyclic rotation around the helicopter, i.e. as a function of their azimuth.
As for why is it on the left, that is because the majority of the population is right handed and the cyclic demands more precise and complex inputs. Therefore it is easier for the pilot to have the cyclic stick in their dominant hand.
Both the cyclic and the collective change the pitch angle of the main rotor blades. The cyclic is typically located in front of the pilot and is controlled with the right hand (presumably because most people are right handed), while the collective is controlled with the left hand.
The pitch angle of the blades is modified with a swashplate. The swashplate can be raised as a whole (collectively, hence the name collective) to increase the pitch everywhere, or in a given direction resulting in different pitch angles during one turn (cyclically, hence the name cyclic).
(image from Wikimedia)